A liquid crystal display panel including a thin film transistor using amorphous silicon as a driving element of liquid crystal is widely used in commercial products such as a monitor of a computer and a liquid crystal television. A manufacturing technique of a thin film transistor using amorphous silicon has been already established and a liquid crystal panel with more than 60 inches has been produced.
Since operation speed of a thin film transistor using amorphous silicon is slow and any further high performance cannot be expected, development of a thin film transistor using polysilicon has been underway. However, a crystallization step is required for forming polysilicon, which leads to cause variation in transistor characteristics and inhibits enlargement of a panel area.
In contrast, an oxide semiconductor material has been attracting attention as a transistor material besides a silicon-based material. As an oxide semiconductor material, zinc oxide or a substance containing zinc oxide is known. Thin film transistors each of which is formed using an amorphous oxide (an oxide semiconductor) having an electron carrier concentration of lower than 1018/cm3 have been disclosed (see Patent Documents 1 to 3).